Carbon-based materials, in general, enjoy wide utility due to their unique physical and chemical properties. Recent attention has turned to the use of elongated carbon-based structures, such as carbon filaments, carbon tubes, and in particular nanosized carbon structures. It has been shown that these new structures impart high strength, low weight, stability, flexibility, good heat conductance, and a large surface area for a variety of applications.
Of growing commercial interest is the use of single-wall carbon nanotubes to store hydrogen gas, especially for hydrogen-powered fuel cells. Other applications for carbon fiber/tubes materials include catalyst supports, materials for manufacturing devices, such as a tip for scanning electron microscopes, electron field emitters, capacitors, membranes for filtration devices as well as materials for batteries.
The formation of carbon filaments through catalytic decomposition of hydrocarbons is known. For example, U.S. Pat. No. 5,165,909 to Tennent et al., disclose the production of carbon fibrils characterized by a substantially constant diameter and a length greater than about 5 times the diameter by continuously contacting metal particles with a gaseous, carbon-containing compound to catalytically grow the fibrils. European Patent 56,004B1 to Yates et al. discloses methods of preparing iron oxides for the production of carbon filaments. U.S. Pat. No. 5,780,101 to Nolan et al. discloses methods of producing highly crystalline nanotubes by the catalytic disproportionation of carbon monoxide in the substantial absence of hydrogen.
U.S. Pat. Nos. 5,872,422 and 5,973,444 both to Xu et al. disclose carbon fiber-based field emission devices, where carbon fiber emitters are grown and retained on a catalytic metal film as part of the device. Xu et al. disclose that the fibers forming part of the device may be grown in the presence of a magnetic or electric field, as the fields assist in growing straighter fibers.
Additional conventional synthesis methods for carbon nanotubes include carbon arc discharge (S. Iijima, Nature, 354, 56, 1991) and catalytic pyrolysis of hydrocarbon (M. Endo, K. Takeuchi, S. Igarashi, K. Kobori, M. Shiraishi, H. W. Kroto, J. Phys. Chem. Solids, 54, 1841, 1993), which generate nanotubes often containing traces of the catalyst particles used to generate them and possessing highly variable dimensions. Synthesis of aligned nanotube by pyrolysis of hydrocarbon with a patterned cobalt catalyst on silica substrate was reported by M. Terrones et al, Nature, 388, 52, 1997 and with iron nanoparticles in mesoporous silica by W. Z. Li et al, Science, 274, 1701, 1996. The successful production of carbon nanotubes in an alumina template by pyrolysis of propylene has been disclosed by T. Kyotani, L. Tsai, A. Tomita, Chem. Mater., 8, 2190, 1996.
Despite efforts in preparing carbon fiber/tubes, limited progress has been realized in controlling the diameters of these materials, particularly controlling the diameter of carbon nanotubes. Accordingly, a need exists for the manufacture of carbon fiber/tubes, in particular nanosized carbon-based fiber/tubes, with improved control over the diameter of these materials.